Power generation using wind-driven windmills (turbines) is usable in areas that have good wind resources and that can benefit from the addition of wind-generated power into a local power transmission system (often referred to as “the grid”). However, wind turbines are relatively unstable power sources that fluctuate with wind conditions and must be properly interfaced to avoid carrying over instabilities into the grid. A wind farm connected to either a weakly supported transmission line or to a relatively small transmission system (such as for an isolated region or island) can inject instabilities in both voltage and frequency of the backbone transmission system because of the gusty and turbulent nature of the wind source. Even on a more robust, interconnected transmission system, such instabilities can create disturbances that propagate through the system.
As wind changes velocity over the area of the wind farm and interacts with individual windmills over varying time periods, and/or turbulent wind flow is created by passing weather systems, the energy output of the wind farm can change very rapidly—over a period of one second or less. This change in energy output of the wind farm is reflected by changes in both frequency and voltage in the transmission grid to which the wind farm is connected. In extreme cases, these fluctuations may become large enough that it is necessary to disconnect the wind farm from the transmission system and simply waste the wind energy. Such conditions have a strong economic impact on a wind farm, which recovers costs only when electricity is being generated. Under less extreme conditions, the shifting winds create energy surges that are reflected in lower-level voltage and frequency disturbances on the transmission system—over a period of 1-2 minutes.
To maintain transmission system stability under these circumstances, compensation is conventionally provided by load-following of the unstable power source with larger capacities of more stable generation units, such as fuel-fired or “thermal” generators. However, such load-following can subject these other units to excessive internal mechanical and thermal fatigue as they absorb fluctuations into their systems over long periods of time. This fatigue adds to both higher operations and maintenance costs, and shortens the overall unit lifetime.
It is also desirable to have a power source provide voltage support to the power transmission grid at the point of its interconnection. Such voltage support enables the power source to contribute to dampening voltage or frequency fluctuations on the transmission line at the point of power injection. In recent years, power flow controllers have been developed to compensate for transmission fluctuations by injecting a power offset varying in voltage and/or phase angle into the transmission system. An example of one type of power flow controller is described in U.S. Pat. No. 5,808,452 to Gyugyi et al. which employs a dc-to-dc converter using the dc voltage produced by a first static inverter connected in shunt with a transmission line to provide parallel reactive compensation to establish the magnitude of a series compensation voltage injected into the transmission line by a second static inverter. However, the various techniques for continuous compensation control are usually associated with the following practical disadvantages: increased circuit complexity and cost, increased losses, and increased harmonic content.
Fluctuations in the power transmission grid can also affect the interconnection of a power source with the grid. Transient conditions such as temporary power outages or flashovers on a transmission line can cause a power sources connected to the grid to become automatically disconnected by its safety circuitry, and would thus require a recloser or other relay type device to reconnect the power source back to the grid once the transient condition has passed. For small-contributor power sources, such as a wind farm, the addition of a recloser or relay device adds an undesirable additional cost to the system. For small power systems, such as an island grid, or a weakly supported interconnected grid where the wind farm represents a major generation source (above 5% of total power), if the wind farm is unable to immediately reconnect to the grid after the fault is cleared (referred to as fault ride through), there may be enough generation/load imbalance to cause the entire grid to shut down due to underfrequency.